echo "hongdada"
a=5;
b=4;
echo "$[a+b]";
echo $[a+b];
echo [ $a + $b ];
echo "[ $a + $b ]";
echo $a + $b ;
echo "expr $a + $b ";
echo 'expr $a + $b ';

r=$((a+b));
echo $r;
((rr=a+b));
echo $rr;
r2=$[a+b];
echo $r2;


# hongdada
# 9
# 9
# [ 5 + 4 ]
# [ 5 + 4 ]
# 5 + 4
# expr 5 + 4
# expr $a + $b
# 9
# 9
# 9


